<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .left{
            float: left;
            width: 200px;
            height: 200px;
            padding: 20px;
            border: 1px solid #000;
            margin: 20px;
            background-color: #ccc;
        }
        .in{
            width: 100%;
            height: 100%;
            border: 1px solid #000;
            background-color: #fff;
        }
        .right{
            float: left;
            width: 200px;
            height: 200px;
            padding: 20px;
            border: 1px solid #000;
            margin: 20px;
            background-color: #ccc;
            box-sizing: border-box;
        }
    </style>
</head>
<body>
    <div class="left">
        <div class="in"></div>
    </div>
    <div class="right">
        <div class="in"></div>
    </div>
</body>
</html>